In response to recent demands for higher performances of displays such as higher definitions and larger screens, active developments have been made in various types of displays.
Representatives of displays currently calling attentions are cathode-ray tubes (CRTs), liquid crystal displays (LCDs), and plasma display panels (PDPs).
A PDP typically has the following construction. Two thin glass plates on which a plurality of electrodes and a dielectric film (a dielectric layer) are formed are placed to face each other, with a plurality of barrier ribs interposed between them. A phosphor layer is formed between every adjacent barrier ribs, and a discharge gas is enclosed between the two glass plates. The two glass plates are hermetically sealed. In such a PDP, power is supplied to the plurality of electrodes, so that an electric discharge occurs within the discharge gas. This electric discharge causes the phosphors to emit light. Unlike a CRT, therefore, a PDP is advantageous in that its depth and weight do not have to be increased accordingly when a screen size of the PDP is increased. Also, unlike an LCD, a PDP is advantageous in that its viewing angle is not limited. In recent years, large-screen PDPs of 50-inch class or larger have already been commercialized.
In a PDP, a protective layer made of magnesium oxide is usually provided on a dielectric film on a glass plate that is positioned to face phosphor layers, in view of protecting the dielectric film against damages.
This protective layer is formed, for example, by spattering. To form a protective layer with good quality, defective factors that may arise at the time of spattering, such as contamination with impurities and generation of static electricity, need to be eliminated. In view of this, moisture is introduced in an atmosphere so as to have a predetermined vapor partial pressure (e.g. approximately 1.5 kPa) where a protective layer forming step is performed. The moisture is considered to reduce impurities suspending in the atmosphere, and also, to reduce static electricity being generated.
The problem is, however, that magnesium oxide possesses the property of absorbing water, and has the property of altering when containing water. Therefore, if the protective layer made of magnesium oxide comes in contact with an atmosphere containing a predetermined amount or more of water vapor, its performances may be degraded.
Also, if moisture is absorbed into the protective layer, the moisture is partially moved toward the phosphor layers during or after the manufacturing of a PDP, causing performances of the phosphor layers to be degraded. Due to this, display performances of the PDP may be degraded.
Further, magnesium oxide also possesses the property of reacting with an atmospheric carbonic acid gas to form magnesium carbonate. If this reaction occurs, too, performances of the protective layer made of magnesium oxide may be degraded.
Also, a large amount of water vapor contained in the atmosphere may cause an erroneous discharge at the time of spattering.
With the problems described above, there still is much room for improvements in manufacturing PDPs to obtain favorable display performances.
The present invention has been made in view of the above problems, and has as an object the provision of a method for manufacturing a PDP with excellent luminous efficiency particularly by forming a protective layer and phosphor layers with good quality. Further, the present invention has as another object the provision of a manufacturing apparatus for such a PDP.